Electronic storage tubes fall into several general categories, including the light outputor direct view storage tube and the electrical output type typically used in conjunction with standard cathode ray tubes for viewing stored images.
A preferred form of electronic output signal storage tube is the so called "beam current reading" type tube (see B. Kazan & M. Knell, "Electronic Image Storage" pp. 123-129, Academic Press 1968) employing a target comprised of a conductive layer and a substantially coplanar insulating layer arranged in a predetermined pattern upon the conductive layer which preferably is a striped pattern. One suitable tube structure is disclosed in U.S. Pat. No. 3,631,294 dated Dec. 28, 1971 and incorporated herein by reference thereto. The present invention constitutes an improvement of the aforementioned U.S. Patent.
The physics of image storage is such that the magnitude of the negative charge on the surface of the insulating layer controls the amount of the beam which reaches the conductive layer. This is exactly analogous to the mechanism of controlling the current flow to the plate in a vacuum tube triode by means of a negative voltage on the control grid. The image formed on the target is stored as a negative charge pattern on the confronting surface of the insulation material. During readout, the insulation surface of the target is at a potential which is everywhere negative relative to the cathode of the storage tube, so that no beam current lands upon the insulation pattern (which may be analogized to the control grid of a vacuum triode, which draws no current since it is at a negative potential relative to the cathode). Thus, image readout is non-destructive in the electron storage tube and the image is retained in the form of the charge pattern formed upon the insulation material during a write mode enabling many reading operations to be performed without destroying the image.
Although some present-day devices to exhibit relatively good image retention times, experimentation still continues in an effort to improve retention time to enable electronic storage tubes to be utilized in applications in which conventional storage tube retention times have been proven to be unsatisfactory.
The electron beam from a vidicon or CRT type gun typically used in a storage tube possesses a beam landing characteristic typified in FIG. 1a wherein the curve A thereon shows target current I.sub.T as a function of target voltage V.sub.T. FIG. 1b shows a typical vidicon type gun structure 1 having control grid G1, acceleration grids G2 and G3, deceleration grid G4, cathode K and target T having a target voltage V.sub.T coupled thereto.
I.sub.max is a function of G1 and G2 voltage while the G3 and G4 voltages determine such factors as beam focus and uniformity of landing on the target (i.e. shading). It can be seen that target current I.sub.T increases with increasing target voltage V.sub.T until it reaches a maximum value I.sub.max after which a drop in the curve occurs due to increasing secondary emission from the target.
The present invention is concerned with electronic output signal storage tubes of the beam current control reading type in which the target is comprised of a pattern of conductive and insulating regions where the conducting regions are connected together and the insulating regions form a "coplanar grid" capable of developing and retaining an electron charge pattern which controls target current to the conducting regions during a read mole in which a conventional scan of an unmodulated electron beam sweeps the target.
In structures of the latter type two of the most significant characteristics are erase time and retention time. Erase time is that time required to "erase" an image so that the target, when swept by an unmodulated electron beam during a typical scanning operation as would be performed during a read mode, will create a totally "black" image. In other words, the target, after erasure, should have its "coplanar grid" provided with a charge pattern which prevents the electron beam from striking the target conducting surface and which develops a uniformly black picture. It is desirable to have erasure time reduced to as low a value as possible in order to permit usage of the electronic storage tube in a wide variety of high-speed applications.
Retention time refers to the ability of an electron storage tube of the latter type to retain an image created thereon regardless of whether the electron storage tube is turned off or has undergone a repeated number of read operations. As retention time with the beam off has been found to be quite long (usually of the order of one week or more) when the electron storage tube is turned off, it has nevertheless been found that the image will fade after repeated read operations.